It would be useful to be able to do a query that returns the tasks that have been closed during a specific time period (e.g., for doing progress reports on a project, etc).

At the moment, this does not appear to be supported, and I haven't found any tasks referencing this functionality. Querying with "Updated After-Before" with Status "Resolved/Wontfix/Invalid" can partially approximate this query, but it depends on tasks not having seen any form of updates after being closed. Since tasks can easily be updated after the time they're closed (e.g., if moved around on a workboard), this is liable to be very imprecise.

Would be nice to have "Closed After"/"Closed Before" functionality similar to the one for Created/Updated,

A version of this query is already used in the "Reports" part of Maniphest, in particular the "Burnup Rate" section (closed by date and closed by week), so hopefully this would not be too difficult to implement.